In some typical wireless communication systems, wireless communication device (e.g. a mobile device) monitors the downlink channel condition and feeds back a corresponding report to the network providing wireless communication system. The report may, typically, comprise a value indicative of a quantized version of the downlink channel condition.
Examples of such reported values include the channel quality indicator (CQI) for High-Speed Downlink Packet Access (HSDPA) applied in the Wideband Code Division Multiple Access (WCDMA) system specified in relation to UTRAN (Universal Telecommunication Radio Access Network) by 3GPP (Third Generation Partnership Project), and the channel state information (CSI) applied in UMTS-LTE (Universal Mobile Telecommunication Standard, Long Term Evolution) also specified in relation to UTRAN (evolved UTRAN).
Typically, the reported value is used by a network node, such as a base station or NodeB or eNodeB (evolved NodeB) in UTRAN terminology, of the network to select which transport format to use for downlink transmissions. Selecting the transport format may comprise varying one or more of the following parameters: transport block size, coding rate, and modulation format. In many systems, the base station does not necessarily have to select the transport format corresponding to the reported value.
According to currently available 3GPP specifications, the wireless communication device shall select and report the value (CQI or CSI as applicable) such that the block error rate (BLER) corresponding to the reported value does not exceed 0.1 (10%) in a static environment.
Generally, it is of high importance that the wireless communication device reports a value (CQI or CSI as applicable) as close to the optimal value (in terms of BLER) as possible to maximize the data throughput. If the wireless communication device reports a higher value than justified by the channel conditions, a higher than targeted block error rate may be the result. On the other hand, if the wireless communication device reports a lower value than justified by the channel conditions, a lower than possible throughput may be the result.
Channel condition indication reporting, of course, matters less if the base station uses its own method to make BLER of the wireless communication device converge to a target value. However, if the convergence method used by the base station is suboptimal, a good channel condition indication mapping may provide improvements. Even if the convergence method used by the base station is well-designed, the reaction time to changes in the environment is typically slow due to the round-trip time between base station and wireless communication device. Hence, during the transient time before a new equilibrium is found by the convergence method of the base station a good channel condition indication mapping may be of benefit. This effect may also useful in scenarios with a large amount of wireless communication devices served by the same base station where each device gets limited time to let the base station convergence method find an equilibrium.
In HSDPA, the CQI value may be determined based on a measured (and typically averaged) Signal-to-Interference Ratio of the Common Pilot CHannel (CPICH SIR). Mapping CPICH (or any other suitable) SIR to CQI is usually done by a SIR-to-CQI mapping table where each entry may be determined by, for example, the mapping function CQI=floor(αSIR+β), where α and β are semi-static values. Typically, SIR is represented by a floating point value and CQI is an integer value between 0 and 15. In this typical case, α and β function to fit the floating point SIR value to the fixed point CQI scale. More generally, the parameter α corresponds to a slope of the SIR-to-CQI mapping function and the parameter β corresponds to an offset of the SIR-to-CQI mapping function.
In UMTS-LTE, the CSI value may, for example, be determined by first determining the mutual information (MI) as MI=ln(γ1SIR+γ2). The mutual information for each resource block may be added together—ΣMI—across a number of resource blocks (wherein the number may be signaled by the network). Then the CSI value may be determined as CSI=floor(γ3ΣMI+γ4), where the parameters γ3 and γ4 can be the same as α and β respectively, or γ1 and γ2 respectively, or different parameters.
In the following description, the HSDPA parameters α and β will be used as examples even though embodiments may be equally applicable to the UMTS-LTE parameters (e.g. γ1 and γ2, and/or γ3 and γ4).
The parameters α and β may be static or semi-static, and may have different values depending on a number of factors, for example, one or more of:                if SIR is above or below a threshold,        a propagation channel dispersion,        a Doppler spread of the channel,        the number of receiver (Rx) antennas of the wireless communication device.        
US 2012/0257568 A1 discloses a method for uplink feedback, where a CQI may be calculated for coordinated dynamic network scheduling. In one embodiment, a ratio of the CPICH signal power for the dominant interfering cell to the CPICH power is mapped to the CQI values.
There is a need for alternative methods of determining channel condition indicators. For example, methods with low calculation complexity would be desirable. Furthermore, methods that model the BLER more accurately in various interference and/or cancellation scenarios would be of benefit.